1. Field of the Invention
The present invention broadly relates to hydraulic cement compositions and methods of their use, and specifically, to additives for retarding the set of hydraulic cement compositions, set-retarded hydraulic cement compositions, and methods of using the compositions for cementing subterranean zones in wells.
2. Description of the Prior Art
In the construction and repair of wells, such as oil, gas and water wells, various operations featuring the use of hydraulic cement compositions are performed. For example, in the completion of a well comprised of a well bore which penetrates one or more subterranean formations, a pipe, such as casing, is disposed in the well bore and a hydraulic cement composition is placed in the annular space between the walls of the well bore and the exterior of the pipe. The cement composition is permitted to set in the annular space whereby a cement sheath is formed therein which bonds the pipe to the walls of the well bore and prevents the undesirable flow of fluids into and through the annular space. Formation of the cement sheath is referred to herein as primary cementing.
In repairing wells, hydraulic cement compositions are often utilized to plug holes or cracks in pipe disposed in the well bore; to plug holes, cracks, voids or channels in the aforementioned cement sheath; and to plug permeable zones or fractures in subterranean formations. Such holes, cracks, voids, channels, and permeable zones are plugged by squeezing hydraulic cement compositions therein which harden and form impermeable plugs. The above procedures are referred to herein as remedial cementing.
High temperatures are frequently encountered in deep subterranean zones. When cementing such zones, the combination of the depth of the zone and high temperatures encountered often results in the requirement that the setting time of the cement composition be extended. That is, the thickening and setting of the cement composition must be retarded or delayed for a time sufficient to permit the cement composition to be pumped into the zone to be cemented. Set retarding additives have been developed and used for this purpose, and such additives have been mixed with well cement compositions in amounts sufficient to delay the setting of the compositions to enable them to be pumped into desired subterranean locations.
While a variety of set retarding additives have been developed and used successfully, they can, and often do, produce erratic results in cement compositions of varying content and/or cause premature gelation of the cement compositions when they are subjected to high temperatures such as those encountered in deep subterranean zones. The high temperatures referred to herein are temperatures up to 250.degree. F. and as high as 500.degree. F. and beyond. Gelation, as used herein, means an increase in viscosity of a cement composition without an increase in compressive strength. Such gelation renders the cement composition difficult or impossible to pump even though the cement composition has not set. In addition, a number of prior art set retarding additives are synthetic polymers which are difficult to produce. Thus, there is a continuing need for improved cement set retarding additives which do not cause premature gelation and which are relatively easy to produce. There is a need for set retarded hydraulic cement compositions containing such additives and methods of using the compositions for conducting cementing operations in wells.